Ferritic heat-resistant steel and method for producing it

ABSTRACT

The invention provides a ferritic heat-resistant steel having excellent high-temperature oxidation resistance, especially excellent steam oxidation-resistant characteristics. In high-Cr ferritic heat-resistant steel, ultra-fine oxide particles having a size of not larger than 1 μm are formed just below the oxide films and formed on the steel base, whereby the adhesiveness between the films and the base is enhanced. The ferritic heat-resistant steel contains Cr in an amount of from 8.0 to 13.0% by weight, and at least one of Rh and Ir in a total amount of from 0.3 to 5.0% by weight.

FIELD OF THE INVENTION

[0001] The present invention relates to ferritic heat-resistant steeland to a method for producing it. More precisely, it relates to ferriticheat-resistant steel suitable for materials for apparatus that are usedunder high-temperature and high-pressure conditions, such as boilers,apparatus in chemical industry, etc., and to a method for producing it.Specifically, it relates to ferritic heat-resistant steel havingexcellent oxidation-resistance at high temperatures, especially steamoxidation-resistance which are not worsened even at high temperatureshigher than 630° C., and having high creep strength which is comparableto that of ordinary steel, and relates to a method for producing it.

BACKGROUND OF THE INVENTION

[0002] In general, heat-resistant steel for use for high-temperatureheat-resistant and pressure-resistant parts of boilers, atomic poweredapparatus and other apparatus in chemical industry is required to havehigh-temperature strength, toughness, high-temperature erosionresistance, oxidation resistance, etc. For those, austenitic stainlesssteel such as JIS-SUS321H, JIS-SUS347H, etc.; low-alloy steel such asJIS-STBA24 (2.1/4Cr-1Mo steel), etc.; and 9 to 12 Cr-type, high-ferritesteel such as JIS-STBA26 (9Cr-1Mo steel) have heretofore been used.

[0003] Above all, high-Cr ferritic steel is widely used in the art, ashaving various advantages. Specifically, it has higher strength andhigher erosion resistance at temperatures falling between 500 and 650°C. than low-alloy steel, and is more inexpensive than austeniticstainless steel. Further, as its thermal conductivity is high and itsthermal expansion is small, high-Cr ferrite steel has good thermalfatigue-resistance while hardly causing scale peeling and stress erosioncracking.

[0004] On the other hand, in the recent thermal electric power plants,boilers are being driven under higher temperature and higher pressureconditions for the purpose of improving the thermal efficiency therein.At present, boilers in those plants are driven under a supercriticalpressure condition at 538° C. and 246 atmospheres, but will be drivenunder an ultra-supercritical pressure condition at 650° C. and 350atmospheres in future. Given that situation, steel for boilers is beingrequired to have extremely high performance, and conventional high-Crferrite steel could no more satisfy the requirements of high oxidationresistance and long-term creep strength, especially steamoxidation-resistance. If the steam oxidation-resistance of boilers arepoor, oxide films will be formed on the inner surfaces of steel pipes ofboilers through which high-temperature steam passes. After having grownto a certain thickness, the oxide films peel off due to thermal stressthat may be caused by the temperature change in boilers, for example,when boilers being driven are stopped, by which pipes will be clogged.Therefore, the prevention of steam oxidation of steel pipes, especiallythe prevention of peeling of oxide films is an important theme.

[0005] As one material capable of satisfying the requirements notedabove, known is austenitic stainless steel. However, austeniticstainless steel is expensive, and its use in commercial plants islimited because of the economic reasons. In addition, because austeniticstainless steel has a large thermal expansion coefficient, its thermalstress to be caused by the temperature change in drive stopping or thelike is large. For these reasons, the use of austenitic stainless steelin plants is problematic because of the difficulties in designing anddriving the plants using it. In view of these, it is desired to improvethe performance of ferritic steel which has a smaller thermal expansioncoefficient and is more inexpensive.

[0006] In order to meet the requirements, recently, various types offerritic heat-resistant steel have been proposed. For example, inJapanese Patent Application Laid-Open (JP-A) Hei-3-097832,Cu-containing, high-Cr heat-resistant steel has been proposed, of whichthe W content is higher than that of conventional steel. Cu is added tothis for improving its high-temperature oxidation resistance. In JP-AHei-4-371551 and Hei-4-371552, high-Cr heat-resistant steel has beenproposed. In this, the ratio of Mo/W is optimized, and Co and B are bothadded (thereto to) thereby increase the high-temperature strength andtoughness of the steel. Even though their high-temperature creepstrength is increased as a large amount of W is added thereto, thosetypes of steel are still problematic in that the decrease in theirtoughness is inevitable. This is because W is a ferrite-forming element,like Mo and Cr, and therefore forms d-ferrite when added in such a largeamount, whereby the toughness of the steel containing W is lowered.

[0007] To solve this problem, it is most effective to form a martensiticsingle phase in steel. For this, for example, reducing the amount of Crto be added to steel has been proposed in JP-A Hei-5-263195, etc.; andadding a large amount of austenite-forming elements such as Ni, Cu, Coand the like to steel has been proposed in JP-A Hei-5-311342,Hei-5-311343, Hei-5-311344, Hei-5-3111345, Hei-5-311-346, etc. These areto improve the toughness of steel by the proposed techniques.

[0008] However, the former steel proposed in JP-A Hei-5-263196 could nothave a sound scale structure since Mo enters the scale consistingessentially of Cr. Therefore, this has poor steam oxidation resistance.To solve this problem, another proposal has been proposed in JP-AHei-8-85847, in which no Mo or only a small amount of Mo is added toW-containing steel. In the steel proposed, W is an essential elementadded thereto for reinforcing it. However, as containing a large amountof Ni and Cu, this steel is still defective, like the steel disclosed inJP-A Hei-5-311342, in that it changes the structure of oxides consistingessentially of Cr₂O₃ and that its steam oxidation resistance is poor.

[0009] On the other hand, the high-Cr ferrite steel disclosed inJP-A-5-311342 and others has a low Al transformation point and a low A₃transformation point, as containing a large amount of Ni, Cu, etc. As aresult, the temper softening resistance of the steel is poor, and, inaddition, carbides and nitrides in the steel rapidly aggregate to givelarge coarse grains therein. Therefore, the long-term creep strength ofthe steel is low. Moreover, Ni, Cu and other elements added to the steelchange the scale layer formed to make it have a brittle structure, likein the heat-resistant steel disclosed in JP-A Hei-5-263196, whereby thesteam oxidation resistance of the steel is worsened.

[0010] As mentioned hereinabove, known is no satisfactory ferriticheat-resistant steel having sufficient oxidation resistance and steamoxidation resistance for use in ultra-supercritical pressure conditionsat high temperatures and high pressures.

SUMMARY OF THE INVENTION

[0011] The present invention has been made in consideration of thecurrent situation noted above, and its subject matter is to provideferritic steel which is free from the drawbacks of conventional ferriticsteel. Specifically, the object of the invention is to provide ferriticsteel, of which the steam oxidation resistance is not lowered even athigh temperatures higher than 630° C., and which has excellent long-termcreep strength.

[0012] In order to solve the problems noted above, the inventionprovides, as in claim 1, ferritic heat-resistant steel capable offorming an oxide film on its surface during use and having good steamoxidation-resistance, which is characterized in that ultra-fine oxideparticles having a diameter of not larger than 1 micron are formed inand/or around the interface between the steel base and the oxide filmformed thereon, to thereby increase the adhesiveness between the oxidefilm and the steel base.

[0013] The invention further provides the following:

[0014] Ferritic heat-resistant steel of claim 1, which contains from 8.0to 13.0% by weight of Cr and contains at least one of Ti and Y addedthereto in a total amount of from 0.01 to 0.30% by weight, as in claim2;

[0015] Ferritic heat-resistant steel of claim 1 or 2, which has acomposition comprising from 8.0 to 13.0% (by weight—the same shall applyherein) of Cr; at least one of from 0.02 to 0.18% of C, from 0.1 to 1.0%of Si, from 0.05 to 1.5% of Mn, from 0 to 0.5% of Ni, from 0 to 4.0% ofW, from 0 to 2.0% of Mo, provided that W+2Mo≦4%, from 0.10 to 0.50% ofV, from 0.02 to 0.14% of Nb, from 0 to 0.1% of N, from 0 to 0.010% of Band not larger than 0.010% of 0; at least one of Ti and Y in an amountof 0.01%≦Ti+Y≦0 30%; and a balance of Fe and inevitable impurities, asin claim 3;

[0016] Ferritic heat-resistant steel of claim 3, which contains at leastone of Co, Rh, Ir, Pd and Pt in a total amount of not larger than 5.0%by weight, as in claim 4;

[0017] Ferritic heat-resistant steel having good steamoxidation-resistance and high long-term creep strength, which containsin an amount of from 8.0 to 13.0% by weight and at least one of Rh andIr in a total amount of from 0.3 to 5.0% by weight, as in claim 5;

[0018] Ferritic heat-resistant steel of claim 5, which contains at leastone of Rh and Ir in an amount of from 0.3 to 5.0% (by weight—the sameshall apply herein) of Rh and from 0.6 to 5.0% of Ir and in a ratio of0.3%≦Rh+(1/2)Ir≦5.0%, as in claim 6;

[0019] Ferritic heat-resistant steel of claim 5 or 6, of which the lathstructure is made fine and the martensite phase is reinforced by atleast one of Rh and Ir added thereto, as in claim 7;

[0020] Ferritic heat-resistant steel of any one of claims 5 to 7, whichcomprises from 0.06 to 0.18% (by weight—the same shall apply herein) ofC, from 0 to 1.0% of Si, from 0 to 1.5% of Mn, not larger than 0.030% ofP, not larger than 0.015% of S, from 8.0 to 13.0% of Cr, from 0 to 4.0%of W, from 0 to 2.0% of Mo, provided that W+2Mo≦4.0%, from 0.030 to0.14% of Nb, from 0.10 to 0.50% of V, from 0 to 0.10% of N, from 0 to0.030% of B, not larger than 0.010% of 0, and from o to 0.050% of sol.Al; at least one of Rh and Ir in a total amount of from 0.3 to 5.0%; anda balance of Fe and inevitable impurities, as in claim 8;

[0021] Ferritic heat-resistant steel having steam oxidation resistance,which contains Cr in an amount of from 8.0 to 13.0% by weight, and atleast one of Pd and Pt in a total amount of from 0.3 to 5.0% by weight,as in claim 9;

[0022] Ferritic heat-resistant steel of claim 9, which contains at leastone of Pd and Pt in an amount of from 0.3 to 5.0% (by weight—the sameshall apply herein) of Pd and from 0.3 to 5.0% of Pt and in a ratio of0.3%≦Pd+Pt≦5.0%, as in claim 10;

[0023] Ferritic heat-resistant steel of any of claim 9 or 10, whichcomprises from 0.06 to 0.18% (by weight—the same shall apply herein) ofC, from 0 to 1.0% of Si, from 0 to 1.5% of Mn, not larger than 0.030% ofP, not larger than 0.015% of S, from 8.0 to 13.0% of Cr, from 0 to 4.0%of W, from 0 to 2.0% of Mo, provided that W+2Mo≦4.0%, from 0.030 to0.14% of Nb, from 0.10 to 0.50% of V, from 0 to 0.10% of N, from 0 to0.030% of B, not larger than 0.010% of 0, and from 0 to 0.050% of sol.Al; at least one of Pd and Pt in a total amount of from 0.3 to 5.0%; anda balance of Fe and inevitable impurities, as in claim 11; and

[0024] A method for producing ferritic heat-resistant steel of any oneof claims 1 to 4, which comprises heating steel at a temperature notlower than 1250° C., subjecting it to plastic working, such as forging,rolling or the like, then immediately keeping it at a temperaturefalling between 1000 and 1150° C. for 1 hour or longer, and thereafterrapidly cooling it to a temperature not higher than its martensitictransformation-finishing point thereby making it have a martensiticstructure, and then heating and tempering it at a temperature fallingbetween 650 and 800° C., as in claim 12.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a cross-sectional view of a steel sample of theinvention, which graphically shows the relational structure of the oxidegrains formed therein and the scale formed on the steel base.

[0026]FIG. 2(A) is a cross-sectional view of a conventional steel samplein which the scale formed is peeling due to the voids formed therein;and FIG. 2(B) is a cross-sectional view of a steel sample of theinvention in which the scale formed is prevented from peeling due to theoxide particles formed therein.

[0027] In those, 1 is an outer scale layer, 2 is an inner scale layer, 3is a steel base, 4 is an oxide particle, and 5 is a void.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] The present invention is characterized by the features mentionedhereinabove. The problems with steel having poor oxidation resistanceare that the oxide film formed on the inner surfaces of steel pipespeels off and deposits in the pipes to clog them, and that the peeledoxide film scatters in steel pipes and erodes the apparatus disposed inthe later zone. From this viewpoint, the present invention has beenmade, and its subject matter is, as so mentioned hereinabove, tohomogeneously form ultra-fine oxide particles having a size of notlarger than 1 μm in and/or around the interface between the oxide filmformed on the surface of a steel base and the steel base just below theoxide film, thereby improving the adhesiveness between the oxide filmand the steel base.

[0029] It is known that, in order to improve the high-temperatureoxidation-resistance of steel, addition of a large amount of Cr or Si tosteel to thereby make the steel have a high Cr or Si content iseffective. However, high-Cr steel is problematic in that d-ferrite isformed therein to lower the toughness of the steel. Therefore, anaustenite-stabilizing element such as Ni, Co, Cu or the like is added toconventional high-Cr steel. However, adding the element isdisadvantageous, since a stable oxide film is difficult to form on thesteel containing the element, resulting in that the oxidation resistanceof the steel is reduced. On the other hand, high-Si steel is alsodefective in that the film formed thereon peels easily, even though itserosion is retarded.

[0030] Given that situation, we, the present inventors have studied thestructure of oxide films formed on various steel samples and also thestructure of the film/steel interface in those samples, and those offilm/steel interfacial structures, and have obtained the followingfindings. Based on those findings, we have completed the presentinvention.

[0031] (1) Fine oxide particles, if existing in/and or around theinterface between a metal base and an oxide film formed thereon,especially in the region just below the film, could be void-fillingpoints in the film and, in addition, could act as a barrier to thegrowth of voids being formed in the interface. Moreover, theadhesiveness between the film and the base is increased by the bridgingeffect of the particles, whereby the film is prevented from peeling.

[0032] (2) For forming such fine oxide particles, it is effective to addto steel an element having high affinity for oxygen, such as Ti or Y, inan amount of from 0.01 to 0.50%. In addition, the element Ti or Y couldtrap oxygen, whereby the diffusion of oxygen into the inside of steel isprevented, and the oxidation speed in steel is much lowered.

[0033] (3) If too large oxide particles are formed in the region justbelow the film, however, they could no more resist the film peeling.Therefore, there is no significant difference between the presence ofsuch large oxide particles and the absence of them.

[0034] (4) In general, adding Ti to ordinary high-Cr ferriticheat-resistant steel produces coarse and large carbides, nitrides andcarbonitrides particles (inclusions) in the steel, whereby the amount ofcarbides, nitrides and carbonitrides of V and Nb that contributes to thestrengthening of the steel is greatly reduced, resulting in that thecreep strength of the steel is lowered. For these reasons, in general,Ti is not added to the steel of that type. However, if the hot workingconditions for the steel could be optimized, the carbides, nitrides andcarbonitrides precipitated in the steel could be dispersed finely andthe creep strength of the steel could be increased.

[0035] Based on these results, the invention provides ferriticheat-resistant steel having both good oxidation resistance and highcreep strength even at high temperatures of 600° C. or higher.

[0036] The constitution of the invention is described in more detailhereinunder.

[0037] <Oxide Precipitates>

[0038] The essential reason for oxide film peeling is thermal stress tobe caused by the temperature change in steel. The thermal stress shallbe greater with the growth of the oxide film on steel (that is, with theincrease in the thickness of the film). When the thermal stress exceedsthe adhesiveness (adhesion strength) between the film and the underlyingsteel base, the film peels from the steel base. Therefore, increasingthe adhesiveness of the film to the steel base is effective forpreventing the film peeling.

[0039] The film adhesiveness is generally increased by densifying theoxide film itself to produce the condition in which pores or voids aredifficult to form in the interface between the film and the steel base.As opposed to this, however, in the present invention, fine particlesare formed in the interface between the oxide film and the steel base,so that they act as a barrier to the film peeling propagation in thefilm/base interface while preventing the film from swelling up.

[0040] The scale peeling-preventing effect of the invention could beinterpreted as follows:

[0041] In the invention, for example, Ti or Y oxides are formed throughinternal oxidation in steel, while the steel is to have a scalestructure composed of an outer scale layer (of Fe oxides) (1) and aninner scale layer (of Fe—Cr oxides) (2) as formed on the surface of thesteel base (3), as in FIG. 1, in which fine oxide particles (4) existaround the scale/base interface.

[0042] In ordinary steel, it is believed that the pores existing in thescale will aggregate in the interface between the scale and the steelbase to give voids (5), as in FIG. 2(A), and those voids (5) will belinked to each other to cause the scale peeling. However, as in FIG.2(B), if fine oxide particles (4) exist around the interface between thescale layers (1) (2) and the steel base (3), especially in the regionjust below the scale (2), they could be void-filling points and evencould act as a barrier to the linking of the voids (5). In addition, theparticles could further act to mechanically bond the scale and the steelbase, whereby the scale is prevented from being swelling up or peelingaway.

[0043] Existing oxide particles having a size of not larger than 1micron, but preferably not larger than 0.5 microns in and/or around theinterface between the oxide film and the steel base prevents the filmfrom peeling, and is effective to attain the intended purpose. However,large particles having a size of 3 microns or larger, if existing in theinterface, are not effective for the intended purpose, but ratherpromote the film peeling.

[0044] <Steel Composition>

[0045] (1) Cr: In general, the oxide film formed on ferriticheat-resistant steel is composed of an outer layer consistingessentially of Fe oxides and an inner layer consisting essentially of Croxides or Fe—Cr oxides. Stabilizing the sound Cr₂O₃ film without peelingit is effective for improving the oxidation resistance of the steel.From this viewpoint, Cr is one essential alloying element in theinvention. Regarding its amount to be added, Cr must be added to steelin an amount of not smaller than 8.0% in order to form a sound oxidefilm. However, if the amount of Cr added is larger than 13.0% much Crwill promote the formation of d-ferrite, whereby the properties of thesteel, including the toughness thereof, are much worsened. For thesereasons, the Cr content of the steel of the invention preferably fallsbetween 8.0 and 13.0%.

[0046] (2) Ti: Ti has high affinity for oxygen. When added to steel in asmall amount, Ti forms fine oxide particles just below the oxide filmformed on steel. Ti easily bonds to not only oxygen but also carbon andnitrogen. Therefore, Ti added in steel alloys well bonds to thoseelements to form its carbides, etc. If the amount of Ti added is smallerthan 0.01%, all Ti will bond to carbon and other elements existing insteel alloys, and could no more form its oxides while the alloys areused. Therefore, it is desirable to add Ti to steel in an amount notsmaller than 0.01%. On the other hand, however, if the amount of Tiadded is too large, Ti oxides formed will be in the form of coarse andlarge particles, and have some negative influences on steel. For thesereasons, the uppermost limit of the amount of Ti to be added may be0.3%.

[0047] In addition, Ti traps oxygen. Therefore, adding Ti to steelprevents oxygen from diffusing into the inside of steel, whereby theoxidation speed in steel is greatly reduced.

[0048] On the other hand, Ti added to steel forms coarse and largecarbides, nitrides and carbonitrides particles (inclusions) in thesteel, whereby the amount of carbides, nitrides and carbonitrides of Vand Nb that contributes to strengthen of the steel is greatly reduced,resulting in that the creep strength of the steel is lowered. For thesereasons, in general, Ti is not added to ferritic heat-resistance steel.However, when Ti-adding steel is heated at temperatures of 1250° C. orhigher, the Ti carbides formed therein will be re-dissolved to formsolid solution. Therefore, if Ti-added steel is subjected topredetermined plastic working, such as forging, rolling, extrusion orthe like, at temperatures falling within that range, and thenimmediately cooled to and kept at temperatures falling between 1000 and1150° C., and thereafter further cooled to temperatures not higher thanits martensitic transformation-finishing point, it may have amartensitic structure with no large and coarse Ti carbides. After this,the steel is tempered at temperatures falling between 650 and 800° C.,whereby fine M23C6 and MC particles are precipitated in the temperedmartensite phase. The creep strength of the thus-worked, Ti-added steelmay be the same as that of the non-worked, Ti-free basic steel. The hotworking is to promote the dissolution of Ti carbides in the steel, andtherefore the hot working temperature is preferably higher. At 1250° C.,the Ti carbides in the steel could be dissolved to form solid solution.Preferably, however, the steel is heated at temperatures not lower than1300° C.

[0049] (3) Y: Like Ti, Y is an element having high affinity for oxygen,and this is effective for positively exhibiting the effect of theinvention. Regarding its amount to be added, it is necessary that, likeTi, Y is added to steel in an amount larger than that capable of bondingto oxygen having dissolved in steel in order that Y added could bondwith further oxygen in actual use of the steel. Therefore, the lowermostlimit of the amount of Y to be added is 0.01%, while the uppermost limitthereof may be 0.3% for the same reasons as those for Ti. Also like Ti,Y traps oxygen. Therefore, adding Ti to steel prevents oxygen fromdiffusing into the inside of steel, whereby the oxidation speed in steelis greatly reduced.

[0050] Regarding those Ti and Y, where the two are both added to steel,the total amount of the two is suitably from 0.01 to 0.3%. If smallerthan 0.01%, they could not sufficiently exhibit the intended effect ofthe invention. However, if larger than 0.3%, they will form coarse andlarge particles. Anyhow, the amount overstepping the range isunfavorable.

[0051] The other elements are added to steel, as in the prior art, forthe purpose of making the steel have the necessary performance such ascreep strength and toughness. For their amount, therefore, referred tois the ordinary knowledge known in the art.

[0052] (4) C: C is an element that forms carbides of various types, MC[as the case may be, in the form of carbonitrides, M(C,N), in which Mindicates an alloying element, and the same shall apply hereunder],M₇C₃, M₆C and M₂₃C₆, and this has great influences on the properties ofsteel. In particular, fine carbide particles of VC, NbC and the like areprecipitated in steel while the steel is used, and they contribute tothe increase in the long-term creep strength of steel. In order thatsuch fine carbide particles are effectively precipitated to strengthensteel, the amount of C to be in steel must not be smaller than 0.06%.However, if larger than 0.18%, too much C will form coarse and largeaggregates of carbides in early stages in use, thereby undesirablylowering the long-term creep strength of steel. For these reasons,suitably, the C content of steel is defined to fall between 0.06 and0.18%.

[0053] (5) Si: Si is an element effective for deoxidizing steel melt andfor improving the high-temperature steam oxidation resistance of steel.However, too much Si lowers the toughness of steel. Therefore, ingeneral, the Si content of steel is defined to fall between 0.01 and1.0% in the prior art. Accordingly, also in the invention, the uppermostlimit of the Si content is 1.0%.

[0054] (6) Mn: Mn is an element to be added to steel for the purpose ofdeoxidizing and desulfurizing steel melt, and this is effective forincreasing the short-term creep strength of steel under high stress. Inorder to attain its effect, Mn must be added in an amount not smallerthan 0.05%. On the other hand, however, if larger than 1.6%, it is knownthat too much Mn lowers the toughness of steel. For these reasons, it issuitable that the amount of Mn to be added falls between 0.05 and 1.5%.

[0055] (7) Mo, W: Mo is effective for solution strengthening of steel.In addition, it stabilizes M₂₃C₆ and increases the high-temperaturestrength of steel. However, if its amount is larger than 2%, Mo promotesthe formation of d-ferrite, while promoting the precipitation andaggregation of M₆C and Laves phases to give coarse and large particles.Therefore, its uppermost limit is defined to be 2%. Like Mo, W is alsosuitable for solution strengthening of steel. In addition, thiscontributes the precipitation of fine particles of M₂₃C₆, whilepreventing carbides from being aggregated to give coarse and largeparticles. Owing to those effects, W greatly increases thehigh-temperature and long-term creep strength of steel. However, iflarger than 4%, too much W often forms d-ferrite and coarse Laves phasesthereby lowering the toughness of steel. Therefore, it is suitable thatthe uppermost limit of W is 4%. Where Mo and W are both added to steel,it is suitable that the total amount of W+2Mo is up to 4%.

[0056] (8) V: V is an element that forms fine carbides, nitrides andcabonitrides particles to contributes to the increase in the creepstrength of steel. In order to attain its effect, V must be added tosteel in an amount not smaller than 0.10%. However, even if added in anamount larger than 0.50%, too much V is no more effective, since theeffect of V is saturated when its amount is up to 0.50%. Therefore, itis suitable that the V content falls between 0.10 and 0.50%.

[0057] (9) Nb: Nb precipitates in steel in the form of its carbides,nitrides and carbonitrides to thereby increase the high-temperaturestrength of steel. In addition, it acts to make the microstructure ofsteel fine, thereby increasing the toughness of steel. Therefore, it issaid that the lowermost limit of Nb to be in steel is 0.02%. However, itis believed that, if Nb is added in an amount of 0.15% or more, it couldnot completely penetrate into the matrix of steel to form solid solutionat normalizing temperatures, and therefore could not sufficientlyexhibit its effect to strengthen steel. Accordingly, it is suitable thatthe Nb content falls between 0.02 and 0.4%.

[0058] (10) N: N is an element to form nitrides and carbonitrides tothereby increase the creep strength of steel. In general, however, ifthe N content is larger than 0.1%, the nitrides formed grow much to givecoarse and large particles, which rather greatly lower the toughness ofsteel. Therefore, the uppermost limit of the N content is preferably0.1%.

[0059] (11) Ni: Ni is an austenite-stabilizing element. It is known thatthis is effective for retarding the formation of d-ferrite andincreasing the toughness of steel. However, if added in an amount largerthan 1%, too much Ni lowers the creep strength of steel. Therefore, theuppermost limit of Ni is preferably 1%.

[0060] (12) B: It is known that B is effective for strengthening theintergranular strength of steel and for finely dispersing M₂₃C₆ carbidesin steel, and that this contributes to the increase in thehigh-temperature strength of steel and is effective for improving thequenchability of steel. It is also known that too much B larger than0.01% forms coarse and large B-containing precipitates therebyembrittling steel. Therefore, it is suitable that the uppermost limit ofB is 0.01%.

[0061] (13) Co, Rh, Ir: Apart from those mentioned hereinabove, Co isknown as an element effective for retarding the formation of δ-ferrite.The recent studies in the prior art are toward the addition of Co tosteel. However, it is known that too much Co lowers the strength ofsteel and even embrittle steel. In general, it is said that theuppermost limit of Co is 5%. Like Co, Rh and Ir are both effective. Co,Rh and Ir may be added to steel in an amount of from 0.3 to 5.0% each.Where two or more of these are added, the total amount is suitably from0.3 to 5.0%

[0062] (14) Sol. Al: Al added to steel essentially acts as a deoxidizerfor steel melt. In steel, Al added exists in the form of its oxides andin any other form. In analysis, the latter is referred to as HCl-solubleAl (sol. Al). So far as steel could be deoxidized by any other elementsadded thereto, sol. Al is not specifically needed. If added in an amountlarger than 0.050% by weight, too much Al will lower the creep strengthof steel. The sol. Al content of steel is suitably from 0 to 0.050% byweight.

[0063] (15) P and S: P and S are both inevitable impurities in steel.These elements have some negative influences on the hot workability ofsteel, the toughness of welded parts of steel, etc. Therefore, theircontent is preferably as small as possible. Specifically, P shall not belarger than 0.030% by weight, and S not larger than 0.05% by weight.

[0064] (16) O: O is also an inevitable impurity in steel. If it locallyexist in steel in the form of coarse and large oxide particles, theparticles have some negative influences on the toughness and otherproperties of steel. In order to ensure the toughness of steel, it isdesirable that the O content of steel is minimized as much as possible.When the O content of not larger than 0.010% by weight, its influence onthe toughness of steel is satisfactorily small. Therefore, the O contentshall not be larger than 0.010%.

[0065] As so mentioned hereinabove, the subject matter of the presentinvention is to form fine oxide particles having a size of not largerthan 1 micron just below the film formed on steel, whereby the film isprevented from peeling off owing to the bridging effect of the oxideparticles. Needless-to-say, therefore, the components constituting thesteel of the invention are not whatsoever limited to those specificallyreferred to hereinabove, so far as the steel attains the object of theinvention.

[0066] In addition, the ferritic heat-resistant steel of the invention,which is characterized by the matters specifically mentionedhereinabove, has been completed on the basis of the following findingsthat have resulted from the data of the detailed studies, which thepresent inventors have made relative to the relationship between theproperty of the steel including its high, long-term creep strength andsteam oxidation resistance, and the chemical components constituting thesteel and the metallic structure (microstructure) of the steel.

[0067] <Long-Term Creep Strength>

[0068] Rh and Ir and also Co are all in the same Group of the PeriodicTable, and they are austenite-forming elements. It has heretofore beenbelieved that, when existing in steel, they greatly lower the Altransformation point of steel thereby lowering the temper softeningresistance of steel.

[0069] However, even if Rh and Ir are added to high-Cr ferritic steelcontaining Mo and W, the Al transformation point of the steel is not somuch lowered. In addition, being different from Co, Rh and Ir added tothe steel do not promote the aggregation and growth of carbides,nitrides and carbonitrides into coarse and large particles. Adding Rhand Ir to the steel makes the martensitic lath structure of the steelfine, while strengthening the martensite phase in the steel. Thisphenomenon is confirmed in ordinary heat treatment of the steel. Thereis found no significant difference in the degree of hardness between thehigh-Cr ferritic steel and conventional steel after they are quenched,but the temper softening resistance of the high-Cr ferritic steel ismuch higher than that of conventional steel. After having beennormalized and tempered, the high-Cr ferritic steel shall have amartensitic texture that contains carbides, nitrides and carbonitridesprecipitated therein. The martensitic structure in the steel tends torecover and soften with the lapse of time at high temperatures higherthan 630° C., which could be prevented by Rh and Ir added to the steel.

[0070] As a result, the long-term creep strength of the steel at hightemperatures higher than 630° C. is greatly increased, and the steelshall have excellent long-term creep strength.

[0071] <Steam Oxidation Resistance>

[0072] Even if Rh and Ir are added to high-Cr ferritic steel containingmuch Mo and W, they do not convert the sound, corundum-type scale layerconsisting essentially of Cr₂O₃ and formed on the steel into aspinel-type structure. Therefore, the scale layer formed on the steel isnot broken, and the steam oxidation resistance of the steel is notlowered even at high temperatures higher than 630° C.

[0073] The effect of Rh and Ir is noticeable when at least any one ofthe two is added to the steel in an amount of from 0.3 to 5% by weight,but preferably when Rh is added thereto in an amount not smaller than0.3% by weight and/or Ir is added in an amount not smaller than 0.6% byweight. However, too much Rh and Ir larger than 5% by weight each, evenif added to the steel, will saturate their effect without augmenting itany more. For these reasons, suitably, the amount of Rh and Ir to beadded is from 0.3 to 5.0% by weight and that of Ir is from 0.6 to 5.0%by weight.

[0074] The effect of these elements noted above can be attained when Rhand Ir are both added to the steel. In the combined addition, however,the amount of the two shall be 0.3%≦Rh+(1/2) Ir≦5.0%, in which % beingby weight, in view of their ability to exhibit and saturate the effect.

[0075] The ferritic heat-resistant steel of the invention can beproduced in any ordinary equipment and process generally employed in theprior art.

[0076] For example, steel is melted in a furnace such as an electricfurnace, a converter or the like, and deoxidizers and alloying elementsare added thereto to control the steel composition. When strictmodulation of the steel composition is specifically needed, the steelmelt may be subjected to vacuum treatment prior to adding alloyingelements thereto.

[0077] The steel melt having been specifically modulated to have apredetermined chemical composition is then cast into slabs, billets oringots in a continuous casting method or a slab-making method, and whichare thereafter shaped into pipes, sheets, etc. Where seamless steelpipes are produced, for example, billets are extruded or forged intothem. For producing steel sheets, slabs are hot-rolled into hot-rolledsheets. The resulting hot-rolled sheets may be cold-rolled intocold-rolled sheets. Where the hot-working is followed by thecold-working such as cold-rolling, it is desirable that the hot-workedsheets are annealed and washed with acids prior to being subjected toordinary cold-working.

[0078] The thus-produced steel pipes and sheets may be optionallysubjected to heat treatment such as annealing or the like, to therebymake them have predetermined characteristics.

[0079] The invention is described in more detail hereinunder withreference to the following Examples, which, however, are not intended torestrict the scope of the invention.

EXAMPLE 1

[0080] Various types of steel each having the chemical composition shownin Table 1 below were produced in a vacuum induction smelting furnacehaving a capacity for 50 kg steel. Ingots produced were hot-forged andhot-rolled into sheets having a thickness of 20 mm, from which testpieces were sampled. In Table 1, Comparative Samples 1, 2 and 3 aresamples of standard steel of ASTM T91, T92 and T122, respectively. TABLE1 Chemical Composition of Steel Samples (wt. %) Type of Steel C Si Mn CrMo W V Nb N Ni Ti Y Others Comparative 0.12 0.45 0.42 8.9 0.99 — 0.210.06 0.05 0.08 — — Sample 1 Comparative 0.12 0.02 0.47 9.1 0.48 1.810.21 0.05 0.04 0.06 — — B: 0.003 Sample 2 Comparative 0.13 0.26 0.6110.3 0.35 2.2 0.22 0.05 0.06 0.31 — — Cu: 0.50 Sample 3 B: 0.003Comparative 0.10 0.05 0.41 9.1 0.50 1.79 0.25 0.05 0.01 — 0.008 Sample 4Sample 1 of the 0.11 0.06 0.45 9.0 0.51 1.81 0.23 0.05 0.01 — 0.03Invention Sample 2 of the 0.11 0.09 0.44 9.0 0.51 1.81 0.23 0.05 0.02 —0.1 Invention Sample 3 of the 0.12 0.07 0.46 9.1 0.48 1.77 0.24 0.050.01 — 0.25 Invention Comparative 0.11 0.06 0.51 8.9 0.50 1.82 0.22 0.050.02 — 0.5 Sample 5 Comparative 0.10 0.10 0.40 9.1 0.51 1.80 0.23 0.050.01 — — 0.006 Sample 6 Sample 4 of the 0.12 0.08 0.50 9.0 0.52 1.810.25 0.05 0.01 — — 0.05 Invention Sample 4 of the 0.10 0.07 0.45 9.00.51 1.78 0.22 0.05 0.01 — — 0.1 Invention Sample 4 of the 0.11 0.070.46 9.1 0.52 1.80 0.24 0.05 0.02 — 0.05 0.05 Invention

[0081] Prior to being subjected to steam oxidation tests for evaluatingtheir steam oxidation resistance, all test pieces were pre-treated forAC normalization at 1050° C. for ______ hours followed by AC temperingat 780° C. for 1 hour. In one steam oxidation test, each test piece waskept heated in a steam atmosphere at 700° C. for 1000 hours, and thethickness of the scale layer formed was measured. In another heat-cycletest, each test piece was heated at the same temperature of 700° C. for96 hours, and then cooled to room temperature, and the heat cycle wasrepeated for a total of 10 times. After the heat-cycle test, the amountof scale peeled off was measured.

[0082] The data obtained in those tests are shown in Table 2, in whichare also shown the presence or absence of oxide particles just below thescale layer and the size of the oxide particles formed. TABLE 2 Data inSteam Oxidation Tests Maximum Diameter of Thickness of Scale OxideParticles Formed Formed in Continuous Amount of Scale Peeled OxideParticles Formed Just Below Scale Layer Type of Steel Heating (microns)off in Cycle Heating (mg) Just Below Scale Layer (microns) ComparativeSample 1 137 158 Yes 3.8 (in laminar distribution Comparative Sample 2201 268 No — Comparative Sample 3 98 106 Yes 2.7 (in laminardistribution Comparative Sample 4 188 215 No — Sample 1 of the Invention93 89 Yes 0.03 Sample 2 of the Invention 72 43 Yes 0.12 Sample 3 of theInvention 89 76 Yes 0.63 Comparative Sample 5 143 131 Yes 1.4Comparative Sample 6 175 187 No — Sample 4 of the Invention 84 38 Yes0.04 Sample 5 of the Invention 66 69 Yes 0.23 Sample 6 of the Invention73 124 Yes 0.05

[0083] As in Table 2, it has been confirmed that Ti and Y added to steelgave fine oxide particles in the region below the scale layer formed, bywhich the amount of the scale peeled off in the heat-cycle test wasreduced. In addition, in the continuous heating test, the thickness ofthe scale layer formed on the steel samples containing any of Ti and Yadded thereto was reduced, from which it is understood that theoxidation speed in those steel samples was retarded.

[0084] Even in the comparative steel samples to which Si had been added,the thickness of the scale layer formed in the continuous heating testwas reduced, and some oxide particles were formed just below the scalelayer. However, in those Si-containing comparative samples, the oxideparticles formed were relatively large and existed inside the Cr2O3layer in laminar distribution. Therefore, in those, it is believed thatthe oxide particles formed rather promoted the peeling of the scalelayer.

EXAMPLE 2

[0085] Sample 2 of the invention in Table 1 was forged at differenttemperatures falling between 1100 and 1400° C., then immediatelyinserted into a furnace at 1050° C. and kept therein for 1 hour, andthereafter cooled with water. After this, the thus-processed sampleswere post-treated for AC tempering at 780° C. for 1 hour. Then, thesewere subjected to a creep rupture test at 650° C. and under 100 MPa. Thedata obtained are shown in Table 3.

[0086] Table 3 —Change in Creep Rapture Strength, depending onhot-working temperature (normalization: 1050° C.×1 hr, tempering: 780°C.×1 hr) Time before Rupture at Heating Temperature 650° C. and underType of Steel (° C.) 100 MPa (hrs) Comparative Sample 1 1200 1013Comparative Sample 2 1200 5931 Comparative Sample 3 1200 6248 Sample 2of the 1100 630 Invention 1200 814 1250 1103 1300 5981 1350 6436 14007124

[0087] As in Table 3, the time before creep rupture of Sample 2 of theinvention, which had been hot-worked at temperatures of 1250° C. orhigher, was prolonged to be longer than that of Comparative Sample 1,T91. In addition, the creep rupture strength of Sample 2 of theinvention, which had been hot-worked at 1400° C., was much increased tobe nearly comparable to that of T92 (Comparative Sample 2) and T122(Comparative Sample 3) The test data support the high creep strength ofSample 2 of the invention.

EXAMPLE 3

[0088] Various types of steel each having the chemical composition shownin Table 4 below were produced in a vacuum high-frequency inductionfurnace having a capacity for 10 kg steel. TABLE 4 W + 2Mo C Si Mn W MoRh + Nb V N P S Cr Rh Ir (1/2) Ir B O So. Al Others Samples of 1 0.090.15 0.53 3.23 0 3.23 0.04 0.18 0.052 the 0.001 0.001 9.25 0 3.77 1.8850.005 0.005 0.003 Invention 2 0.14 0.33 0.99 2.65 0.43 3.51 0.06 0.250.002 0.025 0.002 9.03 1.23 1.05 1.756 0.003 0.003 0.012 3 0.08 0.78 —2.83 0.22 3.27 0.06 0.22 0.062 0.003 38.01 11.8 3.16 0 3.16 — 0.0080.028 4 0.16 0.02 0.21 3.02 0.06 3.14 0.08 0.19 0.002 0.001 0.005 10.4 02.04 1.02 0.003 0.009 0.001 5 0.13 0.54 0.55 1.56 0.75 3.06 0.03 0.280.018 0.014 0.002 8.85 2.11 0 2.11 0.004 0.001 — 6 0.07 0.14 1.32 0 1.663.3 0.006 0.24 0.042 0.024 0.006 8.92 0.37 2.65 1.695 0.006 0.008 0.015Compara- 1 0.11 0.42 0.56 — 0.96 1.92 0.07 0.21 0.051 Ni 0.06 tive 0.0140.006 8.67 — — — — N/A 0.012 Samples 2 0.22 0.51 0.43 0.57 1.03 2.63 —0.35 0.041 Ni 0.15 0.011 0.006 12.12 — — — — N/A 0.021

[0089] Each steel melt was cast into ingots having a diameter of 70 mm,which were then hot-forged at a temperature varying from 1250° C. to1000° C. into sheets having a square of 45 mm×45 mm and a length of 400mm. Then, these were cold-rolled at a temperature varying from 1100° C.to 900° C. into sheets having a square of 15 mm×15 mm.

[0090] Samples Nos. 1 to 6 of the invention in Table 4 were thereafterkept at 1100° C. for 1 hour and then normalized by air cooling, or werekept at 800° C. for 1 hour and then tempered by air cooling.

[0091] On the other hand, Comparative Samples 1 and 2 in Table 4 weresubjected ordinary post-heat-treatment. Briefly, these were kept at 950°C. for 1 hour and then normalized by air cooling, or were kept at 750°C. and then tempered by air cooling. Comparative Samples 1 and 2 had achemical composition of ASTM-A213-T91 and DIN-X20CrMoWV121,respectively.

[0092] Test pieces were sampled out of those eight samples, and testedfor the high-temperature creep strength and the steam oxidationresistance.

[0093] [High-Temperature Creep Strength]

[0094] The test pieces were subjected to a creep rupture test, for whichthe test condition is mentioned below. Test Piece: diameter 8.0 mm gaugelength 40 mm Test Temperature: (1) 650° C., (2) 700° C. Stress: (1) 140MPa, (2) 120 MPa Measured Matter: Time before Rupture

[0095] [Steam Oxidation Resistance]

[0096] The test pieces were subjected to a steam oxidation test, forwhich the test condition is mentioned below.

[0097] Test Atmosphere: steam atmosphere at 700° C.

[0098] Test Time: 1000 hours

[0099] Measured Matter: Thickness of scale formed

[0100] The data obtained in those tests are shown in Table 5. TABLE 5Mean Thickness of Scale Formed in Steam Time before Time beforeOxidation Creep Rupture Creep Rupture (μm) (hrs) (hrs) 700° C. × 1000650° C., 140 MPa 700° C., 120 MPa hrs Samples of 1 3542 187 61 theInvention 2 3216 251 70 3 3733 316 73 4 4308 269 68 5 3884 364 52 6 4336402 77 Comparative 1 65 0.5 151 Samples 2 52 1.3 66

[0101] As in Table 5, the time for creep rupture of all Samples 1 to 6of the invention at 650° C. and under 140 MPa was longer than 3000hours, and that at 700° C. and under 120 MPa was longer than 100 hours.In those Samples 1 to 6 of the invention, the mean thickness of thescale layer formed in the steam oxidation test at 700° C. for 1000 hourswas not larger than 77 μm.

[0102] On the other hand, the creep rupture strength of ComparativeSamples 1 and 2 was much inferior to that of Samples 1 to 6 of theinvention, as in Table 5. Regarding the steam oxidation resistance, thethickness of the scale layer formed in Comparative Sample 1 was about 2times that in Samples 1 to 6 of the invention. This means that the steamoxidation resistance of Comparative Sample 1 is poor.

[0103] From the test results mentioned above, it is confirmed the steamoxidation resistance of the ferritic heat-resistant steel of theinvention is not lowered even at high temperatures higher than 630° C.and that the steel has high creep strength.

EXAMPLE 4

[0104] Various types of steel each having the chemical composition shownin Table 6 below were produced in a vacuum high-frequency

[0105] induction furance having a capacity for 10 kg steel. C Si Mn W MoW + 2Mo Nb V N P S Cr Pd Pt Pd + Pt B O sol.Al Samples of 1 0.09 0.030.48 2.67 0.15 2.97 0.03 0.2 0.048 the Invention 0.021 0.001 8.87 3.25 03.25 0.003 0.002 0.002 2 0.12 0.51 0.82 3.01 0.32 3.65 0.09 0.21 0.0070.003 0.001 9.02 0 3.17 3.17 — 0.006 0.014 3 0.07 0.33 0.41 3.23 0 3.230.07 0.29 0.061 0.008 0.003 8.65 2.32 0.23 2.55 0.002 0.009 0.035 4 0.110.12 0.2 2.03 0.58 3.19 0.06 0.37 0.053 0.017 0.002 9.33 2.45 0.51 2.960.006 0.008 0 5 0.15 0.34 — 0.65 1.33 3.31 0.07 0.19 0.002 0.023 6E−0410.8 1.22 0.38 1.60 0.003 0.003 0.001 6 0.09 0.39 0.83 0 1.53 3.06 0.060.25 0.047 0.002 0.001 9.49 0.56 2.35 2.91 0.008 0.007 0.032 Comparative1 0.11 0.42 0.56 — 0.96 1.92 0.07 0.21 0.051 Ni 0.06 Samples 0.014 0.0068.67 — — — N/A 0.012 2 0.22 0.51 0.43 0.57 1.03 2.63 — 0.35 0.041 Ni0.15 0.011 0.006 12.12 — — — — N/A 0.021

[0106] Each steel melt was cast into ingots having a diameter of 70 mm,which were then hot-forged at a temperature varying from 1250° C. to1000° C. into sheets having a square of 45 mm×45 mm and a length of 400mm. Then, these were cold-rolled at a temperature varying from 1100° C.to 900° C. into sheets having

[0107] a square of 15 mm×15 mm.

[0108] Samples Nos. 1 to 6 of the invention in Table 6 were thereafterkept at 1100° C. for 1 hour and then normalized by air cooling, or werekept at 800° C. for 1 hour and then tempered by air cooling.

[0109] On the other hand, Comparative Samples 1 and 2 in Table 6 weresubjected ordinary post-heat-treatment. Briefly, these were kept at 950°C. for 1 hour and then normalized by air cooling, or were kept at 750°C. and then tempered by air cooling. Comparative Samples 1 and 2 had achemical composition of ASTM-A213-T91 and DIN-X20CrMoWV121,respectively.

[0110] Test pieces were sampled out of those eight samples, and testedfor the high-temperature creep strength and the steam oxidationresistance.

[0111] [Steam Oxidation-Resistance]

[0112] The test condition is mentioned below. Test Piece: diameter 8.0mm gauge length 40 mm Test Temperature: (1) 650° C., (2) 700° C. Stress:(1) 140 MPa, (2) 120 MPa Measured Matter: Time before Rupture

[0113] The data obtained in those tests are shown in Table 7. TABLE 7Average Thikness Average Thikness Average Thikness Time before Timebefore of Scale Formed of Scale Formed of Scale Formed Creep Creep inSteam Oxida- in Steam Oxida- in Steam Oxida- Rupture (h) Rupture (h)tion (μm) tion (μm) tion (μm) 650° C., 140 MPa 700° C., 120 MPa 625° C.× 1000 h 650° C. × 1000 h 700° C. × 1000 h Samples of 1 1211 121 22 3143 the Invention 2 1765 106 23 37 45 3 1553 129 31 35 38 4 1638 113 3642 51 5 1247 170 29 38 44 6 1450 155 32 48 57 Comparative 1 65 0.5 82136 151 Samples 2 52 1.3 45 62 66

[0114] In case of samples 1˜6, thickness of the scale layer formed, isless than 36 μm (625° C.×1000 h), less than 48 μm (650° C.×1000 h) andless than 57 μm (700° C.×1000 h). It was found that each steel of thesamples 1˜6 has superior steam oxidation-resistance at the hightemperature of over 630° C. and is extremely stable.

[0115] Needless-to-say, the invention is not whatsoever limited by theembodiments illustrated hereinabove. For its details, the inventionshall encompass any and every change and modification not oversteppingits scope.

[0116] As has been described in detail hereinabove, the presentinvention provides ferritic heat-resistant steel having excellent steamoxidation resistance and creep strength characteristics. The creepstrength of the steel of the invention is at least comparable to orhigher than that of conventional steel. The steel of the invention isuseful for high-temperature heat-resistant and pressure resistant partscapable of being widely used in various industrial fields, for example,for those of boilers, atomic powered apparatus and other apparatus inchemical industry. For example, the steel may be used for pipes, sheetsfor pressure containers, turbines, etc.

What is claimed is:
 1. Ferritic heat-resistant steel capable of formingan oxide film on its surface during use and having good steamoxidation-resistance, which is characterized in that ultra-fine oxideparticles having a diameter of not larger than 1 micron are formed inand/or around the interface between the steel base and the oxide filmformed thereon, to thereby increase the adhesiveness between the oxidefilm and the steel base.
 2. Ferritic heat-resistant steel as claimed inclaim 1, which contains from 8.0 to 13.0% by weight of Cr and containsat least one of Ti and Y added thereto in a total amount of from 0.01 to0.30% by weight.
 3. Ferritic heat-resistant steel as claimed in claim 1or 2, which has a composition comprising from 8.0 to 13.0% (byweight—the same shall apply herein) of Cr; at least one of from 0.02 to0.18% of C, from 0.1 to 1.0% of Si, from 0.05 to 1.5% of Mn, from 0 to0.5% of Ni, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided thatW+2Mo≦4%, from 0.10 to 0.50% of V, from 0.02 to 0.14% of Nb, from 0 to0.1% of N, from 0 to 0.010% of B and not larger than 0.010% of 0; atleast one of Ti and Y in an amount of 0.01%≦Ti+Y≦0.30%; and a balance ofFe and inevitable impurities.
 4. Ferritic heat-resistant steel havinggood steam oxidation-resistance and high long-term creep strengh, whichcontains at least one of Co, Rh, Ir, Pd and Pt in a total amount of notlarger than 5.0% by weight.
 5. Ferritic heat-resistant steel as claimedin claim 1, which contains Cr in an amount of from 8.0 to 13.0% byweight and at least one of Rh and Ir in a total amount of from 0.3 to5.0% by weight.
 6. Ferritic heat-resistant steel as claimed in claim 5,which contains at least one of Rh and Ir in an amount of from 0.3 to5.0% (by weight—the same shall apply herein) of Rh and from 0.6 to 5.0%of Ir and in a ratio of 0.3%≦Rh+(1/2)Ir≦5.0%.
 7. Ferritic heat-resistantsteel as claimed in claim 5 or 6, of which the lath structure is madefine and the martensite phase is reinforced by at least one of Rh and Iradded thereto.
 8. Ferritic heat-resistant steel as claimed in any one ofclaims 5 to 7, which comprises from 0.06 to 0.18% (by weight—the sameshall apply herein) of C, from 0 to 1.0% of Si, from 0 to 1.5% of Mn,not larger than 0.030% of P, not larger than 0.015% of S, from 8.0 to13.0% of Cr, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided thatW+2Mo≦4.0%, from 0.030 to 0.14% of Nb, from 0.10 to 0.50% of V, from 0to 0.10% of N, from 0 to 0.030% of B, not larger than 0.010% of 0, andfrom 0 to 0.050% of sol. Al; at least one of Rh and Ir in a total amountof from 0.3 to 5.0%; and a balance of Fe and inevitable impurities. 9.Ferritic heat-resistant steel having steam oxidation resistance, whichcontains Cr in an amount of from 8.0 to 13.0% by weight, and at leastone of Pd and Pt in a total amount of from 0.3 to 5.0% by weight. 10.Ferritic heat-resistant steel as claimed in claim 9, which contains atleast one of Pd and Pt in an amount of from 0.3 to 5.0% (by weight—thesame shall apply herein) of Pd and from 0.3 to 5.0% of Pt and in a ratioof 0.3%≦Pd+Pt.≦5.0%.
 11. Ferritic heat-resistant steel as claimed in anyof claim 9 or 10, which comprises from 0.06 to 0.18% (by weight—the sameshall apply herein) of C, from 0 to 1.0% of Si, from 0 to 1.5% of Mn,not larger than 0.030% of P, not larger than 0.015% of S, from 8.0 to13.0% of Cr, from 0 to 4.0% of W, from 0 to 2.0% of Mo, provided thatW+2Mo≦4.0%, from 0.030 to 0.14% of Nb, from 0.10 to 0.50% of V, from 0to 0.10% of N, from 0 to 0.030% of B, not larger than 0.010% of 0, andfrom 0 to 0.050% of sol. Al; at least one of Pd and Pt in a total amountof from 0.3 to 5.0%; and a balance of Fe and inevitable impurities. 12.A method for producing ferritic heat-resistant steel of any one ofclaims 1 to 4, which comprises heating steel at a temperature not lowerthan 1250° C., subjecting it to plastic working, such as forging,rolling or the like, then immediately keeping it at a temperaturefalling between 1000 and 1150° C. for 1 hour or longer, and thereafterrapidly cooling it to a temperature not higher than its martensitictransformation-finishing point thereby making it have a martensitictexture, and then heating and tempering it at a temperature fallingbetween 650 and 800° C.